Filters are almost inevitably required for electrical signals processing. In human body, there are many diagnosis-valued signals which are presented as differential signal, e.g. electrocardiogram, and electroencephalogram, etc.. However, upon recording these physiological signals the measured signals are often corrupted by low-frequency interference. The interferences are caused by the respiration and motion of the subject as well as the differential dc voltage due to the polarization of the electrodes. As a result, the measured signals are degraded or the physiological interpretations are disrupted. Thus, suppressing the low-frequency interferences become important and is the first step in the preprocessing of physiological signals.
In the literature, there are many excellent digital signal processing techniques to remove the low-frequency interference. When the magnitude of the low-frequency interference is large, the resolution of signals in analog-to-digit conversion is limited, and the converted data have not enough precision for physiological interpretations. Hence, the interference should be attenuated by an analog highpass filter before performing analog-to-digit conversion. A novel idea to solve the precision-limited problem is to consider an additional highpass filtering function in a preamplifier. Several circuits which feed the output signals back to the input stage via the integrating network have been developed to realize this idea. However, the integrating network only forms a first-order highpass filter in the preamplifier. The first-order highpass filter filter having an attenuation of -20 dB/decade is not good enough to suppress the high-intensity low-frequency interference. In order to obtain a desired better suppression of unwanted interference, a new circuit is presented in this invention for the realization of a second-order differential highpass filter.